Among synchronous motors, a salient-pole synchronous motor having a rotor including permanent magnets as salient poles and disposed rotatably inside a stator having excitation coils of a plurality of phases for producing a revolving magnetic field is well known. In such a salient-pole synchronous motor, it is necessary that the field magnetic flux distribution due to the salient poles of the rotor, which interacts with the stator having an armature flux distribution of a sinusoidal waveform, be a sinusoidal waveform. One of the measures taken to set up such a magnetic field distribution of sinusoidal waveforms requires that the form of the pole face of each magnetic pole be decided so that EQU G.sub.l =G.sub.min /cos .theta. (1)
where G.sub.l is the gap length between the inner surface of the stator and the pole face of each magnetic pole of the rotor, G.sub.min is the minimum gap length along the axis of the magnetic pole, and .theta. is an angle to the right or to the left from the axis of the magnetic pole.
As illustrated in FIG. 1A, when the magnetic poles of a salient-pole synchronous motor are constituted only by permanent magnets, a rotor 2 rotatably disposed inside a stator 1 needs only permanent magnets 6 attached in the form of salient poles to the outer circumference of a spider 5 attached to a motor shaft 4, and hence the rotor 2 can be easily assembled. However, forming the permanent magnet 6, in general, is difficult, and forming a salient pole having a pole face 3 of the shape capable of establishing a magnetic flux distribution of a sinusoidal waveform is very difficult.
Accordingly, as illustrated in FIG. 1B, a widely employed rotor has a construction in which the projecting end face of a permanent magnet 6a is formed in a flat plane or in a plane of a circular arc, which can be easily formed, and a yoke piece 7 having a surface of a form capable of establishing the above-mentioned magnetic flux distribution of a sinusoidal waveform is attached to the projecting end face of the permanent magnet 6a. Since the yoke piece can be easily formed, this construction is capable of establishing a magnetic flux distribution of a sinusoidal waveform. However, this construction is complicated when assembled, reduces the mechanical accuracy and strength of the rotor, and requires an increased manufacturing cost.